Vibrational scrubbing of wafers, such as ultrasonic or megasonic scrubbing operations, is a well-known, non-contact and brushless technique for removing particles from a semiconductor wafer. During a typical vibrational scrubbing operation, a plurality of wafers is immersed in a process bath, such as comprising a suitable liquid medium, to which vibrational energy is applied. In an ultrasonic operation, the vibrational energy is typically applied at frequencies less than about 400 kHz, while in a megasonic operation the frequencies may range from 700 kHz to 1.2 MHz. High intensity sound waves generate pressure fluctuations that produce millions of microscopic bubbles in the liquid medium. These bubbles rapidly form and collapse (a mechanism called cavitation), producing shock waves that impinge on wafer surfaces. These shock waves displace or loosen particulates that may be present on such wafer surfaces.
A need that arises in these vibrational processes is being able to accurately and consistently maintain an appropriate level of vibrational energy. For example, damage to semiconductor features may occur if the level of vibrational power imparted during a cleaning cycle is excessive. Conversely, if the power setting is too low, then particle removal may be degraded.
It is further desirable to measure the vibrational energy for supporting basic tasks, such as process checks of a vibrational cleaning system that may be performed from time to time. Another desirable capability would be determining effects of different process tools in the process bath. These effects could be baselined and reduced if, for example, one had the ability of measuring and maintaining a desired level of vibrational energy notwithstanding the presence of any such different process tools in the process bath. More importantly, it is desirable to be able to determine the actual degree of sonic vibrational energy delivered to various locations on a surface of a wafer being cleaned in the sonic bath.
Known pressure-sensing devices, such as may comprise a pressure sensor positioned at a distal end of a wand, commonly suffer from inaccuracies due to variations in the vibrational energy pattern that is formed in the process bath. Attempts to overcome the inaccuracies of use of a test wand involve the measurement of vibrational energy at numerous locations in the sonic bath and plotting of the energy distribution on graphs. However, at megasonic frequencies, the wand can affect the energy levels at the measuring location. Further, unless there are products to be cleaned in the sonic bath at the time of measurement, the actual energy delivered will be different from the measured energy. For example, the vibrational energy pattern may be affected based on the physical structures that may be present in the process bath, such as the presence or absence of wafers, inter-wafer spacing, the physical characteristics of a mounting structure for the wafers, the configuration of the pressure-sensing device, the presence or absence of the pressure-sensing device, etc.
Another known technique for determining the level of vibrational energy in the process bath may require the use of a test wafer. That is, a wafer especially prepared with a predefined amount and type of particles. The test wafer is then subjected to various levels of vibrational energy and evaluated after each test to determine particle removal efficiency. This technique may suffer from various limitations such as the following: 1) in practical implementations only certain types of particles may be prepared on the wafer (e.g., nitride or carbide-based particles); 2) adhesion forces between the wafer surface and the particles may be highly variable depending on the shape of the particles, the specific mechanism used for adhering the particles, etc.; 3) the process tank will be contaminated with particles after each test; 4) application of the particles to the surfaces of the test wafer is typically performed manually and is highly variable depending on the specific operator performing the application, and the variation from operator to operator may exist even when using a standard set of instructions; 5) a common technique for applying the particles may require burdensome and careful handling (e.g., dropping nitride particles onto a wafer spinning at a relatively high speed); 6) manufacturing and handling of the test wafer may involve contamination of tools that will require burdensome cleansing before reuse; and 7) inability to identify situations in which the presence of excessive vibrational energy might damage a wafer.